The influence of Strzelin Quarry Lakes on small reservoir retention resources in the regional catchments

The paper presents the results of the analysis of the volume of water retained in Strzelin Quarry Lakes (SQLs). The volume of retained water was estimated by using the computational method, where the proposed reduction factors were determined with the use of DTM (digital terrain model). 2.6 hm3 of water was retained in seventeen Strzelin Quarry Lakes, of which 1.2 hm3 in the Ślęza River catchment (3 quarry lakes), and 1.4 hm3 in the Oława River catchment (14 quarry lakes). The obtained data of the volume of water retained in SQLs were compared to the balance of the water retention capacity of water reservoirs in the catchments of the Ślęza River (0.809 hm3), part of the WR08 Bystrzyca balance catchments (16.190 hm3) and in the catchments of the Oława River (2.782 hm3), part of the WR09 Nysa Kłodzka balance catchment (104.960 hm3). Inclusion the volume of water retained in Strzelin Quarry Lakes in the small scale water retention (reservoirs and ponds) balance would increase the volume of retained water by 156.0% in the Ślęza catchment (by 7.8% in the WR08 Bystrzyca balance catchment) and by 49.5% in the Oława catchment (by 1,3% in the WR09 Nysa Kłodzka balance catchment). Under favorable hydrogeological and geomorphological conditions water reclamation of the excavations may be one of the main aspects of increasing the retention capacity of the catchment, what is particularly important in areas characterized by low water resources.


Materials and methodology
The subject of the study are the Strzelin quarries situated in Lower Silesia (Poland, Central Europe) in Strzelin County, in the area of Strzelin Hills and Lipowe Hills [90][91][92] , in the catchments of the Oława and Ślęza Rivers, in the basin of the Oder River ( Fig. 1) 93,94 . The name Strzelin Quarry Lakes (SQLs) refers to the flooded quarries of cohesive rock material located in Strzelin County.
The Strzelin Hills consist of granitoides (granite, granodiorite, tonalite), gneiss, mica slates, quartzite, quartzite-sericite slate, amphibolite, calcareous/flint rocks, marble, and basalt, while Lipowe Hills contain gneiss, amphibolite, biotite-amphibolite slate, calcareous/flint rocks, granitoid, and basalt, partly covered by sedimentary rocks: silt, clay, and loess 28,[95][96][97][98] . This fostered the exploitation of rock material, and, as a result, led to the presence of over 80 active or closed quarries of various sizes and depths. Quarry lakes have formed in some of them 14,32,[52][53][54]95 .     53,54 was used to determine the maximum depth of the reservoir in the flooded quarry. Tests were conducted on cohesive rock quarries, while other forms of mining and quarrying activities, e.g. sand and gravel quarry, clay quarry were omitted. The analyses were conducted on quarry lakes that retain water permanently, of a maximum depth exceeding 2 m, and a surface area over 1000 m 2 . Additionally, small, periodically drying ponds in close quarries and sumps in regularly drained active quarries were not analysed, either. Due to the mining technology used in Strzelin quarries, it was assumed that their vertical structure was similar (steep vertical walls of the pit, and relatively flat bottom, transport ramps in smaller quarries), and that they varied in terms of the depth and surface area of the pits 14,26,95,106,107 .
The volume of water retained in quarry lakes was calculated as the product of multiplication of the surface area of the given reservoir and its average depth, with use of Eq. (1). The averaged depth of quarry lake was calculated based on the maximum depth of the quarry lake (D max ) and the reduction factor (RF), with use of Eq. (2).
where: V is the estimated volume of water retained in quarry lake (m 3 ), QLS is the area of water surface in the quarry lake (m 2 ), AD q is the average depth of quarry lake (m), D max is the maximum depth of the quarry lake (m), RF is the reduction coefficient to reduce the maximum depth of the reservoir to the averaged value.
The RF coefficient was determined empirically based on DTM (Fig. 3) of the basin of quarry lake Strzelin 14,32 that was created based on LiDAR ALS data collected on the 2012-04-27 as part of the ISOK (Informatyczny System Osłony Kraju, in English: Nation Protection IT System) project 14,32 . The averaged depth of the quarry lake Strzelin was calculated with use of Eq. (3) (calculations were performed for various stages of filling the pit with water (ordinates of the water level in the reservoir)) and Eq. (4). For each calculation instance, the individual reduction factor (IRF) was calculated from Eq. (5). The average reduction factor for actual values (ARF n1, m1 ) and the average reduction factor for actual and prognosed data (ARF n2, m2 ) were determined as the arithmetic mean value. Then, the RF reduction factor was calculated with use of Eq. (6). The calculations and values of the RF coefficient are presented in Table 1. where: AD m is the average depth (m), VBO is the volume of water between ordinates (m 3 ), WSBO is the surface area of the water reservoir on the higher ordinate (m 2 ).
where: IRF is the individual reduction factor (-), AD n,m is the average depth (m), D max is the maximum depth of the water reservoir (m).
where: RF is the reduction factor (-), ARF n1,m1 is the average reduction factor for actual data (-), ARF n2,m2 is the average reduction factor for actual and prognosed data (-). The area of granite quarries situated in the town Strzelin is located on the watershed between the catchments of Ślęza and Oława Rivers, with a larger part in the catchment of the Ślęza River 93,94 . The analysed quarry lake Strzelin (shown in Fig. 2 as quarry lake 1) is drainless, and from the geographic point of view (according to the hydrological map of Poland 93,94 ) is located in the catchment of the Oława River. However, it is partly supplied from the direct catchment (located in the catchment of Oława and Ślęza Rivers) and by transfer of water from the other pits (located in the catchment of the Ślęza River) 14,32 . As a result, quarry lake Strzelin was often assigned to the catchment of Ślęza.
The total volume of water retention in balance catchments (WR08 Bystrzyca and WR09 Nysa Kłodzka) and small-scale reservoir and pond retention in the catchments of the Oława and Ślęza Rivers was determined based on the inventory of the objects discussed above and the presented strategic and planning documentation related to water management 64,72,108 .

Results and discussion
As a result of the inventory, it was found that the retention capacity of large reservoirs in the WR08 Bystrzyca balance catchment had a total volume of 16.19 hm 3 and the small-scale pond and reservoir retention was 5.333 hm 3 (reservoirs: 3.910 hm 3 , and ponds: 1.423 hm 3 ) 64,72 . In the Ślęza catchment, the total volume of small-scale reservoir and pond retention was 0.809 hm 3 (reservoirs: 0.552 hm 3 , ponds: 0.257 hm 3 ), where the volume retained in closed quarry pits amounted to 0.018 hm 364, 72 . The inventory of balance catchment WR09 Nysa Kłodzka revealed a total retention volume in large reservoirs of 104.96 hm 3 and a total volume of small-scale reservoir and pond retention of 4.122 hm 3 (reservoirs: 2.039 hm 3 , ponds: 2.083 hm 3 ) 64,72 . The total volume of small-scale Seventeen quarry lakes were found in the area of Strzelin County, 6 of which were located in active quarries. Most of the Strzelin Quarry Lakes (14) are situated in the catchment of the Oława River, and only 3 of them are located in the catchment of Ślęza River (Fig. 2, Table 2). They are characterised by relatively small water surface areas, ranging from 0.11 to 3.62 ha (1.30 ha on the average), but their average maximum depth is 16.0 m (ranging from ~ 3 to ~ 40 m). The largest and deepest quarry lakes are Górka Sobocka (3.62 ha, 40 m) and Strzelin (2.67 ha, 30 m) ( Table 2, Fig. 4). The volume of water retained in individual SQLs ranges from ~ 3 thousand m 3 to 918 thousand m 3 ( Table 2, Fig. 4). The total volume of water retained in SQLs amounts to 2.635 hm 3 , of which 1.262 hm 3 are retained in the catchment of the Ślęza River and 1.373 hm 3 in the catchment of the Oława River ( Table 2, Fig. 5). The largest amount of water is retained in quarry lake Górka Sobocka (0.918 hm 3 ) and quarry lake Strzelin (0.335 hm 3 ) ( Table 2, Fig. 4). Both of them are located in the catchment of the Ślęza River, (Fig. 2) which belongs to the balance catchment WR08 Bystrzyca. One should remember that in active quarries, mining works are still conducted, which leads to an increase in the volume of the pit and its potential retention capacity. At the same time, some of the water from quarry lakes located in active quarries may be pumped out in order to exploit deeper parts of the deposit.
The comparison of the volume of water retained in SQLs with the volume retained in water reservoirs and ponds in the catchments of the Oława and Ślęza Rivers and the balance catchments WR08 Bystrzyca and WR09 Nysa Kłodzka (Fig. 5) revealed a significant increase in the volume of water retained as part of small-scale reservoir and pond retention. It should be noted that only some of the SQLs were included in the balance of small-scale reservoir and pond retention, while most of these objects was omitted 14,32,64,72 . Taking into account the volume of water retained in SQLs located in the Oława River catchment in the balance of water retained in reservoirs and ponds in this catchment (Fig. 5) resulted in an increase in the volume of water retained in the said catchment by approx. 49.5%, and in the balance catchment WR09 Nysa Kłodzka by approx. 1.3% (Fig. 6). On the other hand, considering the amount of water retained in SQLs located in the Ślęza River catchment (Fig. 5), led to an increase in the amount of water retained in the catchment by approx. 156%, and in the WR08 Bystrzyca balance catchment by approx. 7.8% (Fig. 6).
The volume of water retained in SQLs (Fig. 5) was not included in the balance of reservoir retention capacity in the catchments of the Oława and Ślęza Rivers. As a result, it is not taken into consideration in the water management plans, programmes and strategies, in particular those that concern water retention and distribution. This leads to the lack of reasonable proposals concerning the use of the water retained in SQLs. Currently, the availability of this water for the economy is quite poor and the water is used to a limited extent. In active quarries (No. 1-3, 10, 11, and 14) some of it is used in technological processes, for cutting rocks, washing aggregates, and in water curtains that reduce dust emission and the sprinklers of the loading shafts and transport roads. Some water is also discharged to the network of ditches and watercourses, periodically increasing their flow volume.    www.nature.com/scientificreports/ Most of the SQLs are located in agricultural or forest areas, but none of them are a source of water supply for agricultural or forest irrigation. Due to their location, the potential use of SQLs in water management of the analysed catchments will be mainly of local importance. However, taking the SQLs into account in the balance of reservoir water retention in the catchments of the Oława (1.373 hm 3 (Fig. 5)) and Ślęza Rivers (1.262 hm 3 (Fig. 5)) will enable to include the actions related to the development and renovation of the infrastructure for distributing water from SQLs to potential places of use in the water management plans, programmes and strategies. The low water levels, and even periodical drying of watercourses that persists in some of the rivers in Strzelin County, as well as the agricultural nature of the region will, in fact, point to agriculture as the main direction of use of waters from SQLs. Considering the limited resources of surface waters in Strzelin County, the water retained in SQLs (which has been used to a limited extent so far) may contribute to improving the availability of surface waters in the county, for example for agricultural purposes. Apart from that, focusing on the potential of the water retention possibilities offered by quarries should translate into the appropriate water management of quarries that are being shut down. This may be quite important in the regional aspect, as Lower Silesia is home to dozens of active quarries as well as hundreds of inactive quarries, some of which are flooded.
The assessment of water quality in selected SQLs was the subject of other publications that discussed, among others, the assessment of the content of biogenic substances 54 and the suitability of waters from flooded mining pits for irrigation in agriculture 52,53 . The selected results and conclusions of the water quality assessment in selected SQLs are quoted below.
The results of the assessment of the content of biogenic substances 54 in the waters of SQLs No. 7,8,9,15, and 17 ( Fig. 2) demonstrate that the average total N content fell into the range of 1.10-3.50 mgN dm −3 and total P in the range of 0.39-1.08 mgP dm −3 . The high concentration of total P in all five analysed SQLs (> 0.1 mgP dm −3 ) and the content of total N (> 1.5 mgN dm −3 ) in SQLs 8 and 9 points to the eutrophication of the waters of the analysed SQLs, which manifests itself in algae blooms. No risk of contamination with nitrates from agricultural sources was found, as the maximum concentration of NO 3 in the analysed SQLs was 1.06 mgNO 3 − dm −3 , and the threat caused by nitrates from agricultural sources occurs at levels above 40 mg NO 3 − dm -3109 . The assessment of the suitability of the waters from the analysed SQLs, as a source of potable water supply for residents revealed that SQLs No. 7, 15, and 17 were classified in the highest category A1, SQL No. 8 in the A2 category (due to increased pH), while the water from SQL No. 9 was unsuitable for consumption by humans (exceeded threshold values of pH) 110 . Due to low oxygenation and excessive concentration of phosphates in the waters of SQLs No. 7,8,9,15, and 17 (Fig. 2) the waters did not meet the requirements for fish life in natural conditions 111 .
The assessment of the suitability of the waters from SQLs No. 3, 6, 8, and 14, and 17 ( Fig. 2) for agricultural irrigation 52,53 [112][113][114] . Due to high pH value, water from SQL No. 3 did not meet the requirements provided in PN-84/C-04635. Due to increased nitrate content in SQL No. 3, and excessive sodium concentration and the value of ECw in SQL No. 17, water from these quarry lakes should be subject to slight to moderate restriction on use for irrigation. According to the guidelines of FAO, the assessment of sodium hazard of irrigation water based on SAR and ECw suggests severe restrictions in using water from SQL No. 4 for irrigation and slight to moderate restriction in using the water from SQLs No. 3, 6, 8, and 17 for irrigation. Due to high natrium content, severe restrictions on using water from SQLs No. 3, 6, 8, and 14, and 17 for surface irrigation should be applied. As a result of high pH, water from SQL No. 3 is subject to high restrictions on use for drip irrigation and water from SQLs No. 6, 8, and 14, and 17 should be subject to moderate restrictions on use for drip irrigation. As far as the concentration of heavy metals is concerned, the water from the analysed SQLs (No. 3, 6, 8, and 14, and 17) met the requirements that allow it to be used for agricultural irrigation, in particular sprinkler irrigation. The poor result of the assessment of the chemical condition in SQLs No. 7,8,9,15, and 17, which resulted mainly from the exceeded threshold values for total P, might seem problematic. In quarry lakes, the sources of this element may be the organic matter supplied to the reservoir from belts of trees and bushes and from the forests that surround the SQLs. It may also be released from bottom sediments, and in SQLs No. 8 and 9 it may likely originate from the pressure from municipal and agricultural wastewater. However, the concentrations of selected water quality indicators in SQLs No. 3, 6, 8, 9, and 14, 15, and 17 were similar to the concentrations found in natural lakes not subjected to load, and decidedly lower than the concentrations found in most mine lakes, in particular Acid Mine Lakes and mine lakes in former metal ore excavation sites [52][53][54] . One may suppose that the water from SQLs should not have a significant negative influence on the water quality in the analysed catchments, however it is recommended to continue and expand the scope of research on water quality in SQLs.

Summary and conclusion
Quarry lakes, as post-mining objects, are very often not included in balances of retention capacity of water reservoirs that are located in the catchments of Polish rivers, which is demonstrated by the analysis conducted for the catchments of the Oława and Ślęza Rivers 64,72,108 . Including the volume of water retained in Strzelin Quarry Lakes in the balances of retention capacity of water reservoirs situated in the catchments of the Oława and Ślęza Rivers resulted in a significant increase in the water resources in retention reservoirs in the Oława catchment (by approx. 50%) and in the Ślęza catchment (by approx. 156%). This confirmed the results of previous study on the influence of the volume of water retained in a single granite quarry in Strzelin 14,32 , on the balance of the retention capacity of water reservoirs, thus being part of the national plan for counteracting the effects of drought 63 . This might improve the water balance in the catchment, especially in the part concerning of water retention 3,6,7,68,70 and mitigate the consequences of water deficits. The water retained in quarry lakes may be used for economic and/or environmental purposes. In the event of very low water levels in small natural water courses (e.g. those that are valuable from the natural point of view), water from quarry lakes may be used to supply the water course periodically e.g. to maintain the environmental flow. However, the water from the quarry lake must be of appropriate quality, so as not to deteriorate the quality of water in the watercourse, and not to endanger its ecosystem. Water from quarry lakes may also constitute a source of water supply for industrial plants, e.g. those that use flowing waters, when the flow rate in the water course falls to a level equal or lower than the environmental flow. In the event of long-term drought, it may also be a source of supplying potable water to residents. During agricultural drought, water from quarry lakes may be used as a source of water for irrigation. It may also serve as a source of water supply for breeding ponds, e.g. to fill the pond or to maintain the water inflow, especially when the water levels in watercourses are low and the losses caused by evaporation and leaks from the pond. This becomes particulay important in agricultural areas, such as Strzelin County 14,32 , especially during droughts and periodical drying of watercourses in the analysed catchments 115 . It is important that the water from quarry lakes should meet the quality requirements for the purpose, for which it will be used. For example, water from selected SQLs meets the requirements for the supply of potable water and of water for agricultural irrigation [52][53][54] .
Thus, if the hydrological, geohydrological, and geomorphological conditions are positive, it seems natural to establish retention reservoirs in closed and reclaimed quarries. This will reduce the construction costs of water reservoirs (the excavation pit will be transformed into the basin of the reservoir) and improve the effectiveness of the realisation of tasks foreseen as part of adapting such areas as agriculture, water management, forestry, and industry to climate changes. Due to that, it is worth including the appropriate provisions that would prioritise the water direction of reclamation of post-mining areas in the relevant planning and strategic documents related to environmental protection, water management, adapting to climate changes, and preventing the consequences of droughts and floods.
The conducted research allowed the Author to formulate the following conclusions: www.nature.com/scientificreports/ 2. Due to the mainly agricultural nature of Strzelin County, it seems that agriculture should be the main user of the waters retained in the SQLs. Adding SQLs to the register of water reservoirs and to the balance of the retention capacity of water reservoirs should result in actions aimed at the reconstruction and development of systems that enable the distribution of water from SQLs to recipients. It should also lead to the development of water management programmes, including counteracting the effects of droughts and adapting to climate changes. This should result in improved availability of water for potential recipients, in particular in agriculture. 3. The concentrations of selected water quality indicators in the SQLs were similar to those noted in natural lakes and water reservoirs not subjected to loads, and lower than concentrations found in mine lakes. The parameters of water in SQLs showed that it might potentially be useful for agricultural irrigation (in particular sprinkler irrigation) and for supplying potable water to residents, even if the total P concentrations were slightly increased, pointing to the eutrophication of the SQLs.

Data availability
All data generated during this study are included in this published article. www.nature.com/scientificreports/